Physiologic low stress angioplasty

ABSTRACT

Systems and methods for dilation of a body lumen, using an expandable dilatation catheter to simultaneously heat and apply pressure to tissue of the lumen and to expand and dilate the lumen including means constructed to produce and/or detect physiological response of the heated tissue to applied pressure.

This is a continuation of application Ser. No. 08/146,452, filed Nov. 1,1993, now abandoned which is a continuation of application Ser. No.07/965,518, filed Oct. 23, 1992, now abandoned, which is a continuationof application Ser. No. 07/809,237, filed Dec. 17, 1991, now abandoned,which is continuation of application Ser. No. 07/589,346, filed Sep. 27,1990, now abandoned, which is a continuation-in-part of application Ser.No. 07/404,483, filed Sep. 8, 1989, now abandoned, which is acontinuation-in-part of application Ser. No. 07/263,815, filed Oct. 28,1988, now U.S. Pat. No. 4,955,377.

FIELD OF THE INVENTION

This invention relates to balloon catheters and similar devices usefulto apply heat within a patient's body, e.g. for angioplasty,hyperthermal treatment of tumors, and other medical procedures.

BACKGROUND OF THE INVENTION

Stenoses in body lumens are often treated by balloon catheterdilatation, sometimes followed or accompanied by the application ofheat. A balloon at the end of a catheter is positioned at theobstruction (in a blood vessel, typically plaque) and inflated. Thepressure of the balloon against the wall of the lumen or obstructingmaterial supplies a force to widen the lumen. A problem that sometimesoccurs with dilatation is damage to the lumen tissue or reocclusionbecause of reaction of the lumen tissue to the dilatation. An example isintimal hyperplasia. Also, plaque material in arteries sometimesfractures or cracks under pressure of the angioplasty balloon, leavingrough surfaces that encourage further deposits, thrombosis andreocclusion.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedapparatus and method for the treatment of stenoses in internal bodylumens by use of pressure and heat in a novel way that can help toovercome these and other problems.

The invention features a system for dilation of a body lumen and amethod of using the system, employing an expandable dilatation catheterconstructed to simultaneously heat and apply pressure to the tissue ofthe lumen and to expand and dilate the lumen, and means constructed todetect physiological response of the heated tissue to applied pressure.The means includes catheter control means responsive to the detectedbehavior of the tissue to control the catheter. Preferred embodimentshave the following features. The means to detect physiological responseis constructed to detect yielding behavior of the heated lumen tissuecontacted by the catheter, and the catheter control means is responsivethereto. The means to detect physiological response is constructed todetect change in the heat transfer characteristic of the tissuecontacted by the catheter and the catheter control means is responsivethereto. Means are provided to prevent contraction of the catheterduring cooling of the lumen tissue following heating.

The catheter control means includes a microprocessor and is arranged toreceive feedbacks indicative of temperature and pressure applied by thecatheter, the control means adapted to regulate heating and pressureapplied by the catheter on the basis of the feedbacks and an algorithmimplemented by the microprocessor.

A timing means is provided, constructed and arranged to provide timingof the duration of the dilatation based upon the physiological responseto the heat and applied pressure.

Preferred embodiments also have the following features. A catheter isemployed that is constructed to heat the lumen tissue by conductive heattransfer through a wall of the catheter exposed to the lumen tissue. Thecatheter is a balloon catheter fillable with an electrically conductiveliquid and the associated heating means for producing the heat comprisesrf electrodes within the balloon and means to apply rf energy thereto ina manner to heat the liquid by I² R losses. The catheter is a fluidinflatable catheter, and the catheter control means is an inflationcontrol means. A means to detect physiological response comprises apressure sensor constructed and arranged to sense the fluid pressure inthe catheter and detect reduction in the pressure that results due topressure-responsive yielding behavior of the heated lumen tissue, thecatheter control means being responsive to the detected change inpressure, to increase the volume of inflation of the catheter.Alternatively, a means to detect physiological response includes avolume sensor indicating change in the inflated volume of the inflatablecatheter.

The inflation control means includes a servo motor-driven syringe pump,preferably including a position transducer for measuring thedisplacement of the syringe pump, thereby to indicate the volume of theinflatable catheter. Means are associated with the inflatable catheterto prevent deflation of the catheter during cooling of the lumen tissuefollowing heating, preferably the means to prevent deflation being afluid check valve. The inflatable catheter includes means for measuringthe temperature of fluid within the inflatable catheter.

Also, preferred embodiments have the following features. The cathetercontrol means comprises a controller constructed to receive signalsindicative of the pressure or volume and the temperature of theinflatable catheter, the controller constructed to control the inflationand temperature in response to the signals, for further treatment.

The system includes display means to provide a read-out indicative ofthe physiological response of the tissue under treatment. The read-outindicates pressure applied by the catheter, heating by the catheter andvolume of the catheter.

In another aspect of the invention, a system for dilation of a bodylumen is provided comprising an inflatable dilatation ballon catheterand associated heating means arranged to simultaneously apply pressureto the tissue and heat via conductive heat transfer from the balloon tolumen tissue, and means constructed to detect physiological response ofthe heated tissue to applied pressure. The means including cathetercontrol means responsive to the detected behavior of the tissue tocontrol the catheter.

Preferred embodiments of this aspect of the invention have the followingfeatures. The catheter control means is adapted to increase theinflation of the balloon in reaction to detected yielding behavior ofthe lumen tissue contacted by the catheter. The means to detectphysiological response is constructed to detect change in the heattransfer characteristic of the lumen tissue and to reduce the heating onthe basis of such detected change. The catheter control means comprisesa under control of a programmed microprocessor. The program of themicroprocessor is adapted to increase an inflation set point of aninflation pressure controller in reaction to feedback from the catheterindicating yielding behavior of the heated lumen tissue. Themicroprocessor is programmed to produce heating of lumen tissue at apressure below normal dilatation pressure.

In the method for dilation of a body lumen employing the system justdescribed a number of features are preferred. The method is used toremodel a lumen. The method is used for angioplasty. A step of theprocedure is terminated after a measured period from the time ofdetection of a physiological response. Timing means are employed,constructed and arranged to provide timing of the duration of thedilatation based upon the physiological response to the heat and appliedpressure. The initial temperature of heating is between about 50° C. and70° C. The catheter is a balloon catheter filled with liquid and thewall of said lumen is heated by heating the liquid, with heat transferby conduction from the liquid across the wall thickness of the balloonto the tissue of the wall with which the balloon is engaged.

The liquid within the balloon is electrically conductive and the liquidis heated by I² R losses as a result of radio frequency electriccurrents applied to the liquid.

In another aspect, the invention features a method of angioplasty, inwhich a catheter is provided having a liquid-expansible dilatationballoon and means for controllably providing heated liquid within theballoon to enable conductive heat transfer from the liquid, through thewall of the balloon. The catheter is inserted into a region of a bloodvessel narrowed by plaque or stenotic tissue, and the balloon isinflated to an initial subdilatation pressure sufficient to cause theballoon to engage the wall surface of the narrowed vessel in aconductive heat transfer relationship without substantially displacingthe wall of the vessel. The temperature of the engaged vessel wall isincreased by conductive heat transfer from heated liquid within theballoon while controlling the inflating pressure, the temperature of theliquid within the balloon and the duration of treatment to cause aphysiological response in which the heated wall of the vessel yields tosaid pressure of said dilitation balloon as a result of softening of thewall produced by conductive heat transfer. Dilatation of the vessel canthus occur under relatively low stress conditions.

In preferred embodiments, the pressure in the balloon is about 2atmospheres or less. Controlling comprises maintaining the temperaturein the range of about 60° to 65° C. The subdilatation pressure isinitially maintained such that the flow of blood is substantiallyblocked but without widening the vessel visibly to the naked eye whenobserving the vessel by fluoroscopy. The initially maintainedsubdilatation pressure is such that the vessel does not widen by morethan about 10%.

In preferred embodiments, the method includes maintaining inflation ofthe balloon while reducing the temperature of the balloon afterdilatation of the vessel. Increasing the balloon temperature to a finaltemperature between 50° C. and 70° C. within about 10 to 15 seconds ofthe inflation to the subdilatation pressure, and holding the finaltemperature for about up to about 60 seconds. Thereafter, the balloontemperature is reduced while maintaining the inflation by terminatingthe heating of the fluid and allowing the balloon to cool for about 15to 30 seconds. The inflation pressure is controlled to prevent exceedingthe subdilatation pressure.

In preferred embodiments the progress of the angioplasty is monitored,and the temperature, pressure or duration of treatment is controlled inresponse to the rate of change in the diameter of the vessel. Thepressure, temperature of the balloon or duration of treatment is reducedif a rapid change in the diameter of the vessel indicative of crackingof the substance of the vessel wall occurs. The pressure, temperature orduration of treatment is reduced if the diameter of the vessel increasesby about 25% or more in less than about 0.5 seconds. Monitoring theprogress includes monitoring the inflation of the balloon byfluoroscopy. Monitoring the progress includes monitoring the change inpressure in the balloon.

In preferred embodiments, a balloon is provided having a diametersubstantially the same as that of healthy portions of the vessel. Aballoon is provided having an axial length slightly greater than theaxial length of the region. A balloon is provided with means for I² Rheating of the inflation liquid.

In many respects, the invention is conceived as an improvement over anyprior art method of treating stenoses with heat that strives (solely byprior experiment) to predetermine a proper dose of heat and pressure tobe applied. The heterogeneous nature of stenoses, e.g., differencesbetween different stenoses and within a stenosis, generally preventsapplication of a predetermined heat treatment since, for some stenosesthe predetermined amount is excessive, tending to damage healthy tissue,while for others it is insufficient, resulting in incomplete dilatation.

An advantage of the invention is that dilatation of stenoses is madepossible using a minimum amount of heat and pressure for the tissueunder treatment, as determined by the physiological response of thetissue itself. This "low stress" approach allows effective dilationwhile reducing the risk of undesirable side effects, such as formationof detrimental cracks or fractures or thermal damage. The use of suchcontrolled heat and pressure as described herein reduces the amount ofmechanical stress and disruption in the tissue being treated, therebyreducing the potential for post treatment restenosis and reocclusion.

In general, the treatment procedure of the invention may incorporate oneor more of the following steps.

1. The practitioner chooses a liquid fillable, heatable balloon with aninflation diameter that approximates the diameter of the healthy vesseladjacent the occlusion (the size choice is similar to conventionalangioplasty). In addition, it may be desirable to select a balloonhaving a length slightly greater than the axial length of the stenosisto be treated, to provide a smooth transition of the treated region tothe healthy region.

2. The balloon is inflated to sub-dilation pressures before initiatingheating. A typical sub-dilatation pressure is one to two atmospheres,however, the actual pressure will depend on factors such as balloonsize. In any case, the pressure is selected such that vessel widening isnot visible to the naked eye under normal conditions when observing theoperation by fluoroscopy. Vessel widening under these conditions isgenerally less than 10% and usually less than 5% increase in lumendiameter. The low pressure inflation enables the balloon to contact theocclusion, with a good heat transfer relationship.

3. The temperature of the balloon may be increased, for example, toabout 60° C., but generally not higher than 70° C. The increase frombody temperature, 37° C., occurs over about 5 to 10 seconds (slowcompared to laser or rf heating) permits accurate control and isgenerally accomplished by turning the heater on and allowing it tofollow its normal heat-up curve. The exact length of time of the balloonheat-up is dependent upon the size of the balloon and heater, etc. Insome cases, heating to lower temperatures, for example, 50° C., producesmovement of the occlusion. Full dilatation is usually achieved attemperatures of between 60°-65° C. The temperature may be monitored bythe heated balloon multiplex technique in which energy is supplied and athermistor monitored on a time-sharing basis.

4. The temperature may be held at about 60°-65° C. for up to 60 seconds(e.g., about 10 to 15 seconds) while the operation is monitored byfluoroscopy and/or by monitoring the pressure within the balloon. Withinthis "hold" period the lumen remodels to a fully dilated state where theballoon takes on its normal inflated shape. The use of a balloon filledwith a liquid that is heated which in turn heats the tissue wall incontact with the balloon by conductive heat transfer is advantageousbecause the heating of the plaque is affected by direct conduction andconvection (the latter allows heated plaque to move which allows heatingof underlying plaque). This enables careful control over the heat inputto the tissue, the temperature at the surface of the tissue and the rateat which the temperature is raised. The operator may monitor the rate ofmovement of the tissue, for example, fluoroscopically or by observingpressure changes in the balloon when a constant balloon volume ismaintained. Slow changes, which may be for example, a balloon pressurechange from 2 atmospheres to about 1.5 atmospheres over the course of 2seconds also indicates that the lumen is being dilated in a gradual wayto avoid cracking or other deleterious effects of excess heat orpressure stress. Fast changes, which may be in some cases, the samepressure change occurring in less than about 0.5 seconds, are oftenundesirable because they indicate excessive stress on the lumen. Inresponse to such a fast change, the operator can suspend heating orpressurizing the balloon.

During the course of the treatment, while observing the effectsfluoroscopically, and/or through monitoring the pressure in the balloon,the operator has the option of increasing or decreasing the temperatureor pressure or the duration of treatment in response to the observedeffects on the tissue. Thus, low stress temperature and pressureprofiles can be implemented by the practitioner so that the treatment istailored to the physical characteristics of the particular nature of theplaque encountered.

5. After the vessel is fully dilated, the balloon heater is turned offresulting in the balloon temperature falling off in a gradual wayallowing time for stress to be equalized. The balloon remains inflatedduring this cool down period which may last about 15 to 30 seconds. Theballoon may also be cooled by circulation of a cooling liquid therein.

6. The balloon is deflated and removed.

7. The procedure, steps 1-6, may be repeated without removal of theballoon, although this is generally not necessary since the lumen aftertreatment exhibits smooth surfaces and has a diameter substantially thesame as the healthy tissue adjacent the occluded area, both of whichfactors may inhibit reocclusion which may be either acute, resultingfrom a flap closing or clot formation; mid-term reocclusion in the 6month range which can result from restenosis or scar tissue response; orlong term reocclusion over a year to 18 months later which results fromfailure to properly treat the underlying disease.

The principle behind various aspects of the invention is to use thelowest possible stress to produce dilatation of a stenosed or occludedlumen, and to achieve this by use of a thermal balloon whose heating,preferably by conductive heat transfer, is closely controlled. Lowstress is achieved by applying relatively low temperature to soften theplaque so only relatively low pressures (1-2 atm) are necessary toremodel it to the desired size. The combination of heat and pressure isbelow that which causes significant post-operative platelet deposition.The type of side effects that occur when excessive stress is placed onthe system include lumen responses such as clotting, intimalproliferation (scarring) and hormonal changes which cause restenosis.Longer term effects include the formation of aneurysms (weakening orthinning of the vessel wall).

Plaque is a heterogeneous material, sometimes non-living, that varieswidely and may include calcified, wax-like, fibrotic, and brittlecomponents. Many components are polymeric in nature and for remodelingsuch a material into a desired shape, in this case, the smooth shape ofhealthy lumen tissue, the plaque may be heated as a polymer resin in anextruder. The heat is applied using a liquid filled heated balloon toprovide a thermal profile which enables the material to shape-change onheating in allowing stress equalization. The cooling profile similarlyis important in determining the characteristics of the remodeled plaqueFor example, cooling too quickly can produce a brittle material.

In the invention, the heat is preferably applied to the tissue byconduction from the wall of the balloon. The hottest part of the plaqueis that which is in contact with the balloon surface. As the plaque isheated, thermo-plastic components soften causing, under the pressureapplication, an axial and radial shift in the position of the occludingmaterial, filling in any unevenness and effectively displacing theplaque to expose new surface that is in turn heated and remodeled in thesame way. The result is a lumen having smooth interior walls and adiameter approximately equal to the diameter of the lumen defined by theadjacent healthy tissue without substantial radial movement of thetissue upon which the occluding material was deposited.

This improved understanding of the softening behavior of different typesof plaque as discussed herein helps determine the optimal technique forchoosing balloon size, temperature, time and pressure. By monitoring theballoon pressure decay curve as the vessel responds to heat, theoperator can tune or "titrate" the additional time, heat and pressurenecessary to increment these variables to the best result. For example,a very fast decay could indicate a split or crack, a medium speed wouldsuggest a melting "realignment" and a slow decay would suggest a slowremodeling. If the applied pressure were removed and the balloonpressure rose, it would suggest the presence of elastic recoil. Inaddition, by monitoring this process with intravascular ultrasound(IVUS) from an adjacent vessel and doing a "frame by frame" replay, themelting and remodeling taking place may be observable.

Other aspects and advantages are discussed in the following descriptionand in the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT

We first briefly describe the drawings.

Drawings

FIG. 1 illustrates schematically, a balloon catheter system according tothe invention.

FIG. 2 is a block diagram of a system for physiologically controlledremodeling according to the invention.

FIG. 3 is a balloon catheter blown up and cutaway to show details of theballoon.

FIGS. 4-4d illustrate dilatation of an obstructed artery.

FIGS. 5-5f illustrate balloon pressure, temperature, and volume forexample dilatation procedures 1-7, according to the invention.

Structure

Specific details of the construction of a heated angioplasty balloon canbe found in the parent to the present application, U.S. Ser. No.263,815, filed Oct. 28, 1988, the entire contents of which is herebyincorporated by reference.

Referring to FIGS. 1 to 3, a balloon catheter apparatus 34 includes anondistendable (substantially inelastic) inflatable balloon 8 (forexample, polyester) at the end of a catheter shaft 10. The apparatusalso includes a pressure source 33 with a syringe pump 2 which may be asyringe pump as illustrated, driven by a controllable double actingdisplacement servo motor 4 including a lead screw and a compressionspring assembly 5. Servo motor 4 provides rotary motion to drive leadscrew 5 which translates the rotary motion into a linear motion andconverts motor torque into a linear force. Linear force is applied tosyringe 2 via a compression spring (not shown) and converts the linearforce into fluid pressure, the syringe being filled with saline orconductive contrast medium which is communicated to the catheter balloon8. Check valve 11 prevents flow of fluids from the balloon catheter tothe syringe 2. The compression spring, between the lead screw andsyringe, increases pressure control resolution. In some embodiments, thepump is adapted to maintain a designated pressure and may be, forexample, a spring fed plunger in a syringe or microprocessor controlledpressure regulator. For inflation, fluid passes from the syringe 2through a line 25, one way check valve 11 (such as a ball check valve),three way valve 12, pressure transducer 9 and through an inflation lumenwithin the catheter 10 that communicates with the interior of theballoon 8. Inflated balloon volume is measured by measuring the pistondisplacement with displacement transducer 6. Linear encoder 3 (FIG. 2)is mechanically coupled to the syringe plunger to sense the position ofthe plunger as indication of balloon volume. Linear encoder 3 processessignals indicative of the plunger position to, for example, convertsignals to the rate of volume change or derivative rate of volumechange. Three way valve 12 connects between the pressure transducer 9and the check valve 11 and includes a port 31 which is used for primingand deflating the balloon with a second syringe 38.

Referring particularly to FIG. 2 (--Electrical, ---- Mechanical, andHydraulic systems being indicated), the apparatus has a control module54 with a controller 71 which controls a servo motor power supply 73 andrf power supply 50. A processor 70 sequences treatment events accordingto program algorithms coordinated with feedback from temperature,pressure and volume sensors. Keyboard 72 allows the user to entertreatment parameters and patient data. A display 74 CRT, printer, stripchart recorder, is also provided. In a preferred embodiment, thecontroller 71 drives the rf power supply 50 and servo motor power supply73 according to feedback from the temperature sensor in balloon catheter35 (via wire 17) and pressure transducer 9 (via wire 15). Controller 71is an analog feedback device that adjusts the signal to the powersupplies according to the signal from the temperature sensor or thepressure sensor. The set point is provided to the controller byprocessor (microprocessor) 70 and the controller attains that set pointbased upon feedback. For example, if the signal from the temperaturesensor (along line 17) shows the temperature is lower than the setpoint, controller would increase the rf power being fed to the thermalballoon catheter but if the temperature sensed indicates a temperaturehigher than the set point then the controller would operate in theopposite direction and would decrease the power being supplied by powersupply 50. The system operates in similar fashion with regard to thepressure or the volume controller. The set point is provided by themicroprocessor and the controller controls the direction and forceapplied to the servo motor to achieve that set point. (The ballooninflation may be controlled by the pressure controller or the volumecontroller.) Processor 70 includes the software that will adjust thecontrol of both pressure and temperature. The pin connector of theinflatable catheter wiring to the console 54 includes a binary code toidentify itself to the processor which, on the basis of such identity,and pre-established specifications, determines the control parameters,algorithm, etc. to control the treatment.

To use the system, the catheter is sized to, for example, match oroversize up to 20% (typically about 10%), the diameter of the adjacentundiseased vessel. The balloon is connected fluidly to the pressuretransducer 9 and the syringe 2 is filled with either saline orconductive contrast medium or a combination and attached and connected(as shown). Second syringe 38, partially filled, is attached to thethree way valve vertical port 31. The valve 12 is set for communicationbetween port 31 and the expandable catheter. In repetitive fashion, theplunger of syringe 38 is withdrawn to draw air out of the catheter 34and then pushed forward to remove air from the catheter balloon andprime the lines between. The three way valve 12 is next adjusted toallow communication between port 31 and the syringe and check valve. Thepriming is repeated. This process displaces air and primes theconnecting tubing for accurate volume measurement during operation. Thethree way valve 12 is then adjusted to allow communication between thesyringe 2 and the deflated catheter 34. The balloon is positioned viaguidewire to the point of treatment and pressure exerted to fill theballoon to a predetermined pressure low, subdilatation pressure dictatedby the program algorithm or user input. The processor cycles thepressure and temperature according to the program algorithm.

Referring particularly to FIG. 1, control module 54 includes within aservo motor power supply 73, an rf power supply 50, temperature controland detection circuit 52, pressure control and detection circuit 56 andvolume detection circuit 58. The module 54, further includes readouts,60, 62, 64, for balloon temperature, pressure and volume, respectively.The readouts allow the operator to monitor the physiological response ofthe tissue under treatment and tailor further treatment as required, andas will be further explained below. Connecting lines 13, 15, 17, 19, 21,deliver signals indicative of volume, pressure and temperature andtransport commands and power to the proper elements for controlling eachof these parameters. Line 13, connects to displacement transducer 6 formonitoring inflation volume; line 15 connects to the pressure transducer9 for monitoring pressure; line 17 connects to a temperature sensor 26in balloon 8; line 19 delivers power to rf heating contacts 22, 24 inthe ballon 8; line 21 connects to the motor 4 for delivery of inflationfluid, and line 23 controls check valve 11. The module 54 includesinternal wiring and micro-processor elements as required for control ofthe apparatus as discussed below.

Balloon catheter 34 includes a balloon 8 (e.g. polyethyleneterephthalate, PET) mounted on nylon catheter shaft 10. The fullyextended diameter of balloon 8, when inflated, ranges from 2 millimetersfor coronary vascular procedures, to 20 or 35 millimeters for treatmentof the prostate, esophagus or colon. The volume of the balloon rangesfrom 1/8 cc for the smallest balloon to 100 cc for the largest balloon.The wall thickness of balloon 8 is about 0.001 inch. Guidewire 46, whichcan extend past the distal end of the catheter, may be used to guide thecatheter through the vascular system or luminal structure. Balloon 8 isfillable with an electrically conductive fluid 36 such as normal saline(0.9 percent NaCl in water), a conductive radiopaque fluid, or a mixtureof saline solution and a radiopaque fluid. A valve (e.g., check valve, aball check valve with a downstream vent) may be provided in theinflation fluid plumbing to avoid backflow when the balloon cools downafter treatment (the backflow is caused by a slight shrinking of theartery, which increases the pressure within the balloon). The exteriorof the balloon is coated with a non-stick coating having a lowcoefficient of friction, such as silicone or polysiloxane.

Annular electrical contacts 22 and 24 inside of balloon 8 are bondeddirectly to the catheter shaft. The spacing between the contacts isapproximately half the length of the balloon, and the spacing from therespective end of the balloon is approximately one fourth the length ofthe balloon, so that the balloon will heat evenly. The contacts 22 and24, connect to opposite poles of current-controlled (constant current)radio-frequency power supply 50 in the control module 54. The balloonalso includes a thermistor 26 for measurements of balloon temperature.Wires for the contacts and thermistor are enclosed within catheter shaft10 along its length, and exit catheter shaft 10 through a lumen, 40which is accessible from inside of balloon 8.

Rf power supply 50 preferably operates at 650 kilohertz, but can be atany frequency within the range of about 100 kilohertz to 1 megahertz.The fluid 36, while selected to have resistive losses, has an electricalimpedance low enough that it will conduct the current supplied by rfpower supply 50 at voltages of about 100 volts or lower, so that therewill be no arcing across insulated wires 18 and 20.

The balloon thermistor 26 connects with temperature control means 52which includes a circuit that permits manual or automatic control of theinternal balloon temperature and can preferably be programmed to carryout a temperature algorithm that varies as a function of time. As willbe discussed further herein, useful programs include step programs andprograms allowing linear or nonlinear increase of temperature.Temperature display means 60 is responsive to the temperature control 52and is preferably a CRT display or alternatively a strip chart recorderwhich allows monitoring of measured balloon temperature as a function oftime.

The pressure control means 56 allows manual or automatic programmedcontrol of the pressure of fluid within the balloon by control of thepower supply powering the servo motor. The pressure is monitored by thetransducer positioned in an inflation fluid conduit, downstream from theinflation pump. The pressure can be viewed by the user on a display 62(CRT screen or a strip chart recorder).

The volume control means 58 also includes a circuit arranged to controlthe power supply of the servo motor to deliver fluid to the balloon. Theballoon volume inflation thus can be controlled either through thepressure controller or volume controller. The volume control means 58monitors the volume of fluid passed to the balloon during inflation ordeflation (by, for example, monitoring a servo motor pump or thedisplacement of a syringe pump). The volume of the balloon may bedisplayed graphically as a function of time on volume control means 64(strip chart or CRT).

Operation

Referring to FIG. 4-4d, balloon catheter 34 is used as a heat andpressure source to, for example, dilate a blood vessel by molding thewall or an obstructing material (like plaque). The balloon volume as afunction of time, during the course of the treatment is illustrated inthe graph above each figure. The blood vessel may be a coronary artery,or a peripheral artery such as an iliac, femoral, renal, carotid, orpopliteal artery. A percutaneous insertion is made with a needle, andguide wire 46 is introduced into the blood vessel 42. Balloon catheter34 follows the wire 46 and is positioned at an obstruction in the arterysuch as a plaque deposit 66 (FIG. 4).

The balloon 8 is inflated to engage the plaque material 66 forming theobstruction (FIG. 4a), but the pressure in the balloon is kept below thenormal pressure required under ambient conditions to widen the vessel toavoid cracking the plaque. Normal dilation pressure means the minimumpressure at which an unheated balloon causes substantial dilation of therespective lumen. The low, subdilatation pressure used initially toengage the plaque material may be, for example, about two atmospheres.In the case of angioplasty, normal dilation pressure is of the order of5 to 10 atmospheres (varies with balloon size). The balloon self-formsaround the irregular surfaces of the obstruction and provides a firmcontact for efficient and even transfer of heat. As the occlusion yields(by virtue of heating and gentle pressure as described below), theballoon expands to maintain even contact with the surface and thepressure falls. The control system monitors the pressure, andtemperature and volume of the fluid in the balloon as a function of timeto determine various physiological conditions and responses totreatment, while the user may visually monitor the parameters.

If balloon 8 contains conductive radiopaque fluid, the location ofballoon 8 can be monitored also by means of fluoroscopy. Balloon 8 isinflated through an internal lumen of the catheter with either saline, aconductive radiopaque fluid, or a mixture of saline and a radiopaquefluid. The type and conductivity of the ionic fluid in the balloon ischosen to optimize the conversion of rf energy into heat. (I² R loss,see U.S. Ser. No. 263,815, incorporated, supra). The system is operableover typical pressures in the range of less than 1 to 17 atmospheres.

With balloon 8 inflated to a low level of pressure and engaging theobstruction, the user (or program) initiates the bi-polar heatingbetween the electrodes 36 (e.g. by depressing a footswitch to start aheating program). Heat is dissipated into the fluid according to theformula P=I² R where P is the power that is dissipated into the fluid, Iis the current that is passed through the electrodes, and R is theresistance of the fluid. The heat from the fluid is conducted across theballoon wall into the surrounding tissue 44. The fluid will heat to thetemperature set by the user or carry out a temperature algorithm. Thetemperature at the balloon surface ranges from 45°-90° C. and istypically from 50° to 70° C., sometimes preferably, around 60°-65° C.The higher temperatures in the range are used for relatively thickdiseased areas, where heat conduction out of the target tissue is highor where deeper penetration or therapeutic effect is desired. Heatingwill continue as the time/temperature algorithm dictates or until theoperator deactivates the program. Typical treatments in the coronaryartery take 15 to 60 seconds including a 5 second temperature increaseand ten seconds to reduce the balloon temperature prior to deflation andremoval.

While heating, the operator may monitor the condition and physiologicalresponse of the vessel under treatment, particularly by observing thevolume and/or pressure of the balloon. When the obstruction is undercertain conditions of heat and pressure, the heterogeneous plaquematerial (usually including fat, fibrogen, calcium) softens, resultingin a sudden change in the allowable volume of the balloon at a given lowpressure (below the pressure needed to crack the obstruction). Analogousto a thermoplastic material, the heat, decreases the yield stress of theplaque and artery under treatment to the point where the stress inducedby the balloon exceeds the yield stress thereby causing a sudden changein balloon volume. The physiological response, the sudden yield of theobstruction, is detected by the system as a sudden change in the volumeor pressure, or rate or change of volume or pressure.

In FIG. 4b, for example, the volume of the balloon is shown, for examplepurposes, to increase slowly as the occluding material elastically(reversibly) expands with gentle heating until it increases suddenlyupon reaching a yield point (y) at time (t_(y)) corresponding to theconditions of pressure and temperature at which the occlusion yields.Thereafter, the volume again increases as the occluding material yieldsplastically (substantially nonreversibly).

As shown in FIG. 4c, after detection of the yield point, the operatordetermines the course of further treatment, which may include changes intemperature or pressure of the balloon, to effect full dilatation of theartery where the continued treatment leads to full expansion of theballoon and artery at a time (t_(d)).

Finally, after the vessel has been fully dilated, the temperature of theballoon is reduced, while the balloon remains inflated. Recycling thetemperature allows the material of the obstruction, the plaque, to bemold-formed by the balloon as it cools and reconstitutes. The interiorwalls of the remodeled lumen are left smooth and with reduced chance ofreocclusion. The temperature is reduced while the balloon is inflated.The balloon is deflated and removed from the body lumen (FIG. 4d).

The following examples 1-7 illustrate possible operating proceduresusing a fluid delivery means adapted to maintain set pressures in theballoon. It is an advantage of the invention that by feed back ofphysiological response during treatment, the operating procedures may betailored for the specific condition of each patient where, for example,the nature of the occluding material (composition, location, morphology)may be different. The occlusion may be treated using the least obtrusivepressure and temperature conditions to dilate the vessel in a mannerthat may discourage hyperplasia of the artery and reocclusion.

In FIGS. 5-5c balloon pressure, temperature, and volume as a function oftime are indicated for the Example procedures 1-6, below. The time(t_(i)) is the initial time at which the balloon is positioned at theobstruction and inflated to contact the obstruction (but below maximumpressure, P_(max), or volume, V_(max)). The time (t_(y)) corresponds tothe time at which yield point of the obstruction material is reached.The time (t_(d)) is the time at which the vessel is fully dilated. Ineach of the examples, the temperature of the balloon is reduced beforethe balloon is deflated and removed at time (t_(f)). It will beunderstood that the balloon temperature decrease may be programmed tofollow a path different from the symmetrically opposite (recycled) pathof the temperature increase, as shown in Examples 1-7.

EXAMPLE 1

Referring now to FIG. 5, the balloon is inflated and maintained at aconstant pressure by a pressure regulator over the course of thetreatment. The temperature is initially increased. The volume of theballoon remains substantially constant until time (t_(y)), when theoccluding material yields, and thereafter volume increases. The lineartemperature program is maintained until the balloon reaches its maximumvolume, at (t_(d)) indicating the vessel is fully dilated. Thetemperature is held constant for a short time and then reduced. Theballoon is deflated and removed (As shown, as temperature is reduced,measured pressure increases slightly as the vessel contracts slightly,the check valve preventing fluid flow and therefore allowing pressurebuildup.)

EXAMPLE 2

Referring now to FIG. 5a, the balloon is inflated and the pressure isheld constant during the course of the treatment. The temperature isincreased until the yield (ty) is detected and is thereafter heldconstant while the volume of the balloon expands to full dilatationvolume (td). The temperature is held constant for a time after fulldilation of the balloon and then reduced. The balloon is deflated andremoved.

EXAMPLE 3

Referring now to FIG. 5b, the balloon is inflated, the temperatureincreased. The pressure is held constant until the yield is detected(ty), at which time the pressure is increased until the balloon is fullydilated (td). The pressure and temperature are held constant for a time.The temperature is then reduced. The balloon is deflated and removed.

EXAMPLE 4

Referring to FIG. 5c, the balloon is inflated to a low pressure and thetemperature is increased until the yield is detected (t_(y)). Thereafterthe temperature is held constant and the pressure increased until theballoon is fully dilated (t_(d)). The pressure and temperature are bothheld constant for a time and the temperature reduced. The balloon isdeflated and removed.

EXAMPLE 5

Referring now to FIG. 5d, the balloon is inflated and the temperature isbrought quickly to a temperature and held constant during the course ofthe treatment. The pressure in the balloon is increased until the yieldis detected (t_(y)). Afterwards the pressure is held constant againwhile the balloon inflates to its maximum dilatation volume (t_(d)). Thetemperature is then reduced. The balloon is deflated and removed.

EXAMPLE 6

Referring how to FIG. 5e, the balloon is inflated and the temperatureincreased until the yield is detected (t_(y)). The temperature is thenheld constant until the balloon is fully dilated (t_(d)). Thetemperature is then increased above the yield temperature (for example,5 degrees) and held for a period of time. The temperature is thenreduced. The balloon is deflated and removed.

EXAMPLE 7

Referring now to FIG. 5f, the balloon is inflated and, initially, thepressure held constant while temperature is increased until the yield isdetected (t_(y)), after which the temperature also is held constant.After the yield the volume is observed to increase until time (t_(s))when the volume plateaus at a volume below the full dilatation volume.Upon detection of the volume plateau, the temperature and pressure areboth increased until a second yield is observed at time (t_(2y)), afterwhich the temperature and pressure are held constant and the balloonexpands to full inflation volume (td). The temperature may be reduced,the balloon deflated and removed.

Example 6 demonstrates the flexibility of determining treatment based ondetection of physiological response. With the apparatus and methodsdescribed herein the device may be adjusted to tailor treatment based onchanging conditions during the procedure.

Other Embodiments Operation

It will be understood that the invention may be practiced otherwise thanas described herein. The principal of the invention is to allow thetissue/lesion to respond at the lowest temperature and pressurenecessary to perform dilatation to minimize effects such as cracking.The tissue responds physiologically, meaning that only when thesoftening point is reached, will the material remodel. Although ingeneral a yield point is detectable as described above, there may becases where a gradual dilatation occurs without an abrupt or detectableyield point being observed. In one other embodiment, the operatormonitors the pressure in the balloon after inflation with a given volumeof inflation fluid to determine the physiological response to treatment.For example, the balloon may be inflated with a volume of inflationfluid such that the balloon engages the obstruction but does not dilatethe lumen. (A screw cranked syringe could be used to deliver a selectedvolume). The obstruction is then heated and the pressure in the balloonmonitored by the system. For example, the operator may inflate theballoon to a sub-dilation level, e.g., 2 atmospheres. Next thetemperature is increased to approximately 60 C.--a level which is highenough to soften the plaque allowing remodeling at low pressure, but notso high as to desiccate or char the plaque. At the yield point of thematerial, the expansion of the balloon is detected by the system as adecrease in pressure (no further inflation fluid is delivered). Afterthe yield point is detected, the system provides for further treatmentif desired, for example, introducing more inflation fluid, adjusting theballoon temperature, etc. Additionally, the rate of decrease in pressuremay be monitored on a display device to determine the condition of thelumen and the progress of treatment.

A further physiological response measured by the system could be achange in thermal characteristics of the tissue. This would be measuredby change in power required to maintain a constant temperature withinthe balloon. A change in thermal characteristics may be measured by thediminution in power required to maintain a constant temperature withinthe balloon, because of a diminished thermal gradient between theballoon wall and the tissue. The power required may also be diminishedby a reduction of blood perfusion through the tissue due to acoagulation of perfusion channels within. The changes in power requiredare sensed at the rf energy supply output, which may be fed back intothe microprocessor for establishing further control.

It will also be understood that programmed increases in temperature andpressure may be linear, nonlinear or step functions. The timing ofoperations may be keyed to the absolute volume, change in volume or rateof change of volume of the balloon.

Heating

Additionally, it is believed most advantageous to heat the tissueprimarily through conductive heat transfer by physically contacting thetissue with a controlled heating element (e.g. an rf heated balloon asdescribed above). Plaque, tumors, intimal hyperplasia, diseased tissue,etc., are generally less vascularized, (i.e, less cooled by blood flow)than surrounding healthy tissue. Thermal conductive heating thereforeprovides higher specificity for heating target tissues while avoidingsubstantive heating (and damage) of healthy, vascularized tissue. Itwill be understood, however, that treatment with less specific heatingmeans, such as heating by direct exposure of the tissue toelectromagnetic radiation (heating in this case is due to non-specificabsorption of photons by all tissue) or rf conduction through the tissue(which heats by inducing electric currents through electricallyconductive tissue and blood) may be configured to take advantage ofphysiological feedback control as described herein.

The chart below compares the laser, RF surface electrodes and directheating modes.

    ______________________________________    COMPARISON OF DIFFERENT METHODS    OF HEATING A VESSEL WITH A BALLOON                    RF surface   Hot Fluid           Laser    electrodes   inside balloon    ______________________________________    Heating  Optical    Electrical   Thermal    Principal             Absorption Resistance   Conduction                        (I.sup.2 R loss in                        tissue)    Control  Power      Impedance of Temperature    Principal             dosimetry  tissue       of balloon fluid             (possibly             acoustic             feedback)    Factors    affecting    tissue heating    Primary  Tissue     Electrical   Thermal             absorption conductivity conductivity    Secondary             Thermal    Thermal      Thermal mass             conductivity                        conductivity (specific heat)    Implications             Heats dark Heats areas  Heats areas    for clinical             tissue pre-                        of highest   that are poor             ferentially                        conductivity heat sinks             Heterogenous                        first . . . such                                     first. This             colors heat                        as healthy   includes             tissue     tissue       plaque             unevenly   containing blood             Burnt tissue                        Plaque may   Will not             keeps heating                        not heat as  overheat             Temperature                        well         beyond preset             is hard to              temperature             control             Can overheat                        Temperature  Healthy vessel                        is hard to   is heated                        control      less                                     Temperature                                     can be precisely                                     controlled                                     Rate of heat                                     rise can be                                     controlled    ______________________________________

The uniformity of the balloon temperature can be encouraged by mixing orcreating a turbulence in the inflation fluid within the balloon. In oneembodiment, this is accomplished by withdrawing a portion of the fluid,for example 25% of the volume, and quickly reinjecting. The reinjectioncan be carried out at a convenient time during or prior to heattreatment. Usually a single reinjection is sufficient to preventtemperature stratification of the inflation fluid, even for relativelylarge balloons of e.g., 30 mm, inflation diameter.

Additionally, the depth of the heating can be controlled by selection ofballoons of different diameters. A balloon with an inflation diameterlarger than the natural diameter of the lumen under treatment (thediameter without occlusion), enhances mechanical contact and compressesthe obstructing material. Fluid flow, such as blood flow within thecompressed tissue is also minimized. All of these factors increase thepenetration depth of heat into the occluding material. With properselection, the heating depth may be controlled to heat treat foroccluding material without substantially heating or damaging theunderlying healthy tissue of the lumen wall. Larger balloons, givingbetter contact and higher compression increase the penetration depth.

Uses

The balloons, as described, can also be used for investigations of themorphology of occlusions. In one procedure, the balloon is positioned atthe obstruction and inflated to make firm contact with the occlusion butthe pressure is kept below that needed for dilation. The balloon isheated, for a period, at a temperature below the yield point of thematerial, but high enough to mold the balloon material around themorphological features of the occlusion. Finally, the balloon is allowedto cool (while inflated) and then withdrawn. Upon reinflation outsidethe body, the balloon takes a molded shape (concentric, eccentric,reflective of calcified or fibrous formations, etc.) influenced by themorphology of the occlusion, and may be studied as a three dimensionalmodel. In another procedure, the balloon is studied after dilatation asdescribed herein. The balloon, after being dilated and cooled will takethe shape of the artery post angioplasty. The topography of the arterypost angioplasty is a useful indication and quantification of thesuccess of the procedure.

It will also be understood that the invention may be used in lumensother than those of the vascular system. For example, dilatation may beperformed in the esophagus, prostate, GI tract, fallopian tube, urinarytract, biliary tree, pancreatic duct, surgically constructed anastomosesor tract accesses to organs (e.g. fistulas) or any lumen constricted byplaque, intimal hyperplasia, a tumor or other obstruction.

In an alternative embodiment for monitoring the temperature of theballoon, the impedance of the saline inflation fluid may be monitored.

Other embodiments are within the scope of the claims.

What is claimed is:
 1. A method of angioplasty, comprising:providing acatheter having a liquid-expansible dilatation balloon and means forcontrollably providing heated liquid within the balloon to enableconductive heat transfer from the heated liquid, through the wall of theballoon, inserting the catheter into a region of a blood vessel narrowedby plaque or stenotic tissue, inflating said balloon to an initialsubdilatation pressure sufficient to cause the balloon to engage thewall surface of the narrowed vessel in a conductive heat transferrelationship without substantially displacing the wall of the vessel,increasing the temperature of the engaged vessel wall by conductive heattransfer from heated liquid within the balloon while controlling theinflating pressure, the temperature of said liquid within said balloonand the duration of treatment to cause a physiological response in whichthe heated wall of the vessel yields to said pressure of said dilatationballoon as a result of softening of the wall produced by said conductiveheat transfer, thereby enabling dilatation of said vessel to occur underrelatively low stress conditions.
 2. In the method of claim 1,controlling the pressure in said balloon to about 2 atmospheres or less.3. In the method of claim 2 is maintaining said temperature in the rangeof about 60° to 65° C.
 4. In the method of claim 1 initially maintainingsaid subdilatation pressure such that the flow of blood through thevessel is substantially blocked without widening the vessel visibly tothe naked eye when observing said vessel by fluoroscopy.
 5. In themethod of claim 1 initially maintaining said subdilatation pressure suchthat the vessel does not widen by more than about 10%.
 6. In the methodof claim 1, after dilatation of said vessel, maintaining inflation ofsaid balloon while reducing the temperature of said balloon.
 7. In themethod of claim 1, increasing the balloon temperature to a finaltemperature between 50° C. and 70° C. within about 10 to 15 seconds ofsaid inflation to said subdilatation pressure, and holding said finaltemperature for a period up to about 60 seconds, and, thereafterreducing said balloon temperature while maintaining said inflation byterminating said heating of said fluid and allowing said balloon to coolfor about 15 to 30 seconds.
 8. In the method of claim 1 controlling theinflating pressure to prevent said pressure from exceeding said initialsubdilatation pressure.
 9. The method of claim 1 further comprisingmonitoring the progress of said angioplasty, and controlling thetemperature, pressure or duration of treatment in response to the rateof change in the diameter of said vessel.
 10. The method of claim 9further comprising reducing the pressure, temperature of said balloon orduration of treatment if a rapid change in the diameter of said vesselindicative of cracking of the substance of said vessel wall occurs. 11.The method of claim 10 further comprising reducing the pressure,temperature or duration of treatment if the diameter of said vesselincreases by about 25% or more in less than about 0.5 seconds.
 12. Inthe method of any one of claim 9 to 11, monitoring said progress byfluoroscopy.
 13. In the method of any one of claims 9 to 11 monitoringsaid progress by monitoring the change in pressure in said balloon. 14.The method of claim 1 in which a balloon is provided having a diametersubstantially the same as that of healthy portions of the vessel. 15.The method of claim 1 or 14 a balloon is provided having an axial lengthslightly greater than the axial length of said region.
 16. The method ofclaim 1 in which the inflatable balloon is provided with means for I² Rheating of said inflation liquid.
 17. A system for dilation of a bodylumen, for use with an expandable dilatation catheter constructed tosimultaneously heat and apply pressure to the tissue of the lumen toexpand and dilate the lumen, comprising:means constructed to detectphysiological response of heated lumen tissue to applied pressure, saidmeans including catheter control means responsive to the detectedbehavior of the heated tissue to control said catheter to enabledilatation of said lumen under relatively low stress conditions.
 18. Thesystem of claim 17 wherein said means to detect physiological responseis constructed to detect yielding behavior of the lumen tissue contactedby said catheter, and said catheter control means is responsive thereto.19. The system of claim 17 wherein said means to detect physiologicalresponse is constructed to detect change in the heat transfercharacteristic of the tissue contacted by said catheter and saidcatheter control means is responsive thereto.
 20. The system of claim 17including means that prevents contraction of said catheter duringcooling of said lumen tissue following said heating.
 21. The system ofclaim 17 wherein said catheter control means includes a microprocessorand is arranged to receive feedbacks indicative of temperature andpressure applied by said catheter, said control means adapted toregulate heating and pressure applied by said catheter on the basis ofsaid feedbacks and an algorithm implemented by said microprocessor. 22.The system of claim 17 including timing means constructed and arrangedto provide timing of the duration of the dilatation based upon thephysiological response to said heat and applied pressure.
 23. The systemof claim 17 in combination with a catheter constructed to heat saidlumen tissue by conductive heat transfer through a wall of said catheterexposed to said lumen tissue.
 24. The system of claim 23 wherein saidcatheter is a balloon catheter fillable with an electrically conductiveliquid and associated heating means for producing said heat comprises rfelectrodes within said balloon and means to apply rf energy thereto in amanner to heat said liquid by I² R losses.
 25. The system of claim 17wherein said catheter is a fluid inflatable catheter and said cathetercontrol means is an inflation control means.
 26. The system of claim 17wherein said means to detect physiological response comprises a pressuresensor constructed and arranged to sense the fluid pressure in saidcatheter and detect reduction in said pressure that results due topressure-responsive yielding behavior of the heating lumen tissue, saidcatheter control means responsive to said detected change in pressure,to increase the volume of inflation of said catheter.
 27. The system ofclaim 26 where said means to detect physiological response includes avolume sensor indicating change in the inflated volume of saidinflatable catheter.
 28. The system of claim 25 wherein said inflationcontrol means includes a servo motor-driven syringe pump.
 29. The systemof claim 28 including a position transducer for measuring thedisplacement of said syringe pump, thereby to indicate the volume ofsaid inflatable catheter.
 30. The system of claim 25 including meansthat prevent deflation of said catheter during cooling of said lumentissue following said heating.
 31. The system of claim 30 wherein saidmeans to prevent deflation is a fluid check valve.
 32. The system ofclaim 25 further including means for measuring the temperature of fluidwithin said inflatable catheter.
 33. The system of claim 25 wherein saidcatheter control means comprises a controller constructed to receivesignals indicative of the pressure or volume and the temperature of saidinflatable catheter, the controller constructed to control saidinflation and temperature in response to said signals, for furthertreatment.
 34. The system of claim 17 further including display means toprovide a read-out indicative of the physiological response of saidtissue under treatment.
 35. The system of claim 34 wherein said read-outindicates pressure applied by said catheter, heating by said catheterand volume of said catheter.
 36. A system for dilation of a body lumencomprising:an inflatable dilatation balloon catheter and associatedheating means arranged to simultaneously apply heat via conductive heattransfer from said balloon to lumen tissue and pressure to said tissue,and means constructed to detect physiological response of heated lumentissue to applied pressure, said means including catheter control meansresponsive to the detected behavior of the tissue to control saidcatheter to enable dilatation of said lumen under relatively low stressconditions.
 37. The system of claim 36 wherein said catheter controlmeans is adapted to increase the inflation of said balloon in reactionto detected yielding behavior of the lumen tissue contacted by saidcatheter.
 38. The system of claim 36 or 37 wherein said means to detectphysiological response is constructed to detect change in the heattransfer characteristic of said lumen tissue and to reduce the heatingon the basis of such detected change.
 39. The system of claim 36 whereinsaid catheter control means comprises a controller under control of aprogrammed microprocessor.
 40. The system of claim 39 wherein saidprogram of said microprocessor is adapted to increase an inflation setpoint of an inflation pressure controller in reaction to feedback fromsaid catheter indicating yielding behavior of the heated lumen tissue.41. The system of claim 39 wherein the microprocessor is programmed toproduce heating of lumen tissue at a pressure below normal dilatationpressure.
 42. The system of claim 36 wherein said inflatable dilatationballoon catheter is an angioplasty catheter, carrying an inflatableangioplasty balloon.
 43. A method for dilation of a body lumen,comprising:employing a system comprising the combination of anexpandable dilatation catheter constructed to simultaneously heat andapply pressure to the tissue of the lumen and to expand and dilate thelumen; and means constructed to detect physiological response of heatedlumen tissue to applied pressure, said means including catheter controlmeans responsive to the detected behavior of the tissue to control saidcatheter to enable dilation of said lumen under relatively low stressconditions.
 44. The method of claim 43 wherein said means to detectphysiological response is constructed to detect yielding behavior of thelumen tissue constructed by said catheter, and said catheter controlmeans is responsive thereto.
 45. The method of claim 43 wherein saidmeans to detect physiological response is constructed to detect changein the heat transfer characteristic of the tissue contacted by saidcatheter and said catheter control means is responsive thereto.
 46. Themethod of claim 43 wherein said dilation is employed to remodel a lumen.47. The method of claim 43 wherein said dilation is employ forangioplasty.
 48. The method of claim 43 including terminating a step ofsaid procedure after a measured period from the time of detection of aphysiological response.
 49. The method of claim 43 employing timingmeans constructed and arranged to provide timing of the duration of thedilatation based upon the physiological response to said heat andapplied pressure.
 50. The method of claim 43 wherein the initialtemperature of heating is between 50° C. and 70° C.
 51. The method ofclaim 43 wherein said catheter is a balloon catheter filled with liquidand the wall of said lumen is heated by heating the liquid, with heattransfer by conduction from said liquid across the wall thickness of theballoon to the tissue of the wall with which said balloon is engaged.52. The method of claim 51, wherein the liquid within said balloon iselectrically conductive and said liquid is heated by I² R losses as aresult of radio frequency electric currents applied to said liquid.